Understanding seasonal sea ice levels

Is so much easier if you can take advantage of expert opinion. So I did some back-of-the-enveloping, then decided to ask someone who would know much more than me.

Who best to ask? WTF: I emailed Mark Serreze the following enquiry:

I have noticed that Bill Chapman’s site at CT/UIUC is currently showing a negative areal anomaly of more than -2.7 Million Km2, which as far as I can ascertain is the largest recorded anomaly for this metric in satellite records.

Can you tell me if the same is true for your method of calculating extent, i.e., if the present anomaly (for the past 5 days, for example) is now larger than it was during the week of minimum extent, Sept. 16th?…

…I am also curious to know how far you think the anomaly might go before next April; is there any chance we might see a total areal anomaly of -3.5Mkm2, or even more?

Never let it be said that scientists aren’t good, decent considerate folk. Heck, these guys have their jobs to do, as well as handle the press, and could do without dumb emails from the likes of the Old man. So I wasn’t surprised when Mark replied, though I was extra pleased by the bonus email from Walt Meier. Here are a couple of extracts from their replies:

Fergus:

… in terms of ANOMALIES, yes, I would not be surprised if we still had a record, both in terms of ice area and extent. Check our last posting, you will see that extent is still way below normal.

Walt: do you have the numbers handy in terms of current ice extent anomalies?

Ice extent and area are increasing only slowly this autumn, the reason being that there is still so much heat in the Arctic Ocean. Will anomalies grow through this winter? I just don’t know. However, the ice we have by April will likely be pretty thin, setting us up for another big loss next summer. Cheers

Mark C. Serreze

And Walt replied, too:

Hi Fergus,

Yes, we have been tracking the extent anomalies relative to the climatology (1979-2000) and indeed it did continue to grow well after the minimum extent was reached on Sept. 16.

In fact, it reached as much as -3.25 million sq km about a week ago. Since then it has moderated some, but it’s still nearly -3 million sq km.*

This isn’t too surprising. There was such a huge area that was ice-free, allowing the ocean to warm up. This means it’s going to take longer to cool to the freezing point allowing ice to form. I would expect that the anomaly magnitiude will decrease through the winter**, but may still stay larger than the previously anomalies of ~1 million sq km we’ve seen the past couple of years. Also, as Mark mentions below, if where there is ice, it will be thinner.

walt

* a quick technical point; the numbers I use are for the CT ‘area’ metric, Walt’s are the NSIDC ‘extent’ anomaly numbers.

**walt emailed to mention the typo, where he put ‘summer’ rather than ‘winter'; it has now been adjusted.

So, the anomaly did increase after the minimum was reached, by both measures. CT is still going downwards, though this could change within a week; to answer a potential disagreement on the earlier thread, my ‘eye’ estimate reads the ‘tale of the tape’ showing an areal anomaly of -2.75 Mkm2 +/- 0.05.

From this, plus my unique method of pseudo-straight-line analysis, I will now estimate the forthcoming seasonal mean extents for Autumn (October-December) and Winter (January-March). This can be compared against the fourth of the top-line graphs at CT – the one with the pretty colours.

The OND mean extent will probably not exceed 10Mkm2, and is more likely to be as low as 9.3Mkm2, +/- 0.2.

JFM should be below 13.5; my guess is 12.8 Mkm2 +/- 0.4. The mean annual extent, calculated at the end of Autumn, will be below 11 Mkm2, perhaps as low as 10.2.

I couldn’t work out how to estimate the Spring mean.

This sets the scene nicely for the bets between William, Eli and the others over at Stoat, for next Summer’s minimum. There is a very good chance that the maximum in March will be down by more than 1 Mkm2, and it could be as much as 1.5. The ice in April will be thin, and will melt or dissipate more rapidly as a result. Next year’s minimum might be very close to this year’s all-time record decline. I don’t know which side of the line it will fall, but I’ll hazard that it will be considerably lower than the 2005 (former) record minimum.

If anyone thinks my estimates are likely to be out by more than 1 Mkm2 either way, I’m willing to consider a small wager…

My sincere thanks go to Mark and Walt, and the excellent people who work at both the NSIDC and the UIUC to produce the data; you’re all heroes.

12 comments

Thanks for the form guide. Nice to see you agree with my much more intuitive grasp of affairs. I’m up to £20 with William, but never bet much more than that in any circumstance since my wife wanted to put £100 on Maori Venture and I said no… (it was about 90/1 at the time).

Can anyone comment on the food chain, the algae that grows on the winter ice and goes into the Arctic Ocean as the ice melts?

Is the algae volume more related to surface area, or to thickness?

I don’t know where the algae lives — top? bottom? throughout the ice?

I’d guess perhaps algae can live throughout the thickness of the ice — get captured during the winter freeze and then grow nicely inside ice once the sun comes up, limited to whatever extent by availability of CO2 for photosynthesis and removal of oxygen waste from the ice — but I”m guessing, and perhaps the algae needs either air or water rather than ice around it to keep that happening).

And I don’t know if the algae is released by melting off the top and bottom surfaces, or if breakup of sheet ice into smaller pieces exposes more algae living inside the ice.

If only the surface(s) of the ice sheet are colonized by algae, then there’s going to be less algae this past year and next year, I’d guess — again, unless the algae grows faster in warmer air or water on such surfaces.

If the volume is normally colonized and algae is growing inside the ice when the light returns — again, with less thick ice there’s less algae to enter the food chain.

I’m just wondering how much of a change in algae going into the ocean is going to happen with thinner and faster-melting sea ice.

And clearly I know nothing about this. I’ve been looking for papers but none I’ve come across address this very simple stuff that may be assumed as known by most readers.

When I plot ice area or ice extent anomalies as standard deviations, it looks like a very strong and statistically significant trend starting ~2005. The last couple of years simply do not belong to the same data population as the 1979-2000 data.

As I eyeball the curve, it looks like a graph of ice falling off a Quonset Hut.

My guess for ice area minima next year is ~2 M KM ^2., I think that warm, salty North Atlantic water is finding its way into the Arctic mid and near surface-waters, and ultimately that is reducing the ice cover. Changes in atmospheric circulation increase ice export, and increased storm mixing help. Decreased albedo is a strong feedback. Less, ice also means more water vapor, which is a very strong feedback.

More water vapor over the Arctic has strong implications for Greenland. Anybody want to bet how much rain Swiss Camp gets in the next decade?

Sorry I haven’t replied recently, Hank; been busy saving the world. I think the algae is a bottom-of -the-ice grower, generally, though there are probably several species with different habits. Does it matter whether the volume is more related to area or thickness? Either way, there’s not much prospect of future flourishing.

As well as the algae, there’s the phytoplankton and even smaller stuff; the bottom of the food chain, and, I believe, a rather important oxygenator.

Aaron; don’t tease; if you have an interesting number, show it. Otherwise, it’s hard to understand where you’re getting ~2MKm2 from: on the face of it, this looks unlikely, unless you have a reason to think otherwise.

It’s important to remember that we’re playing with the numbers to a certain extent here, rather than looking at the physical objects, and, as such, we are not taking into account all of the variables which will have an effect on seasonal sea ice levels next year. (I’d suggest anyway that it’s too far away to have a strong reason to predict a further loss in the range of 30-50% in top of this year’s record. OTOH, I think it would be difficult to justify predicting a Summer minimum for next year anywhere near the long-term figure.)

As for the Greenland precipitation; this is a difficult one to call, due to lack of records, but I’d guess that melt season rainfall on the GIS will have a variety of effects. I’ll think about this.

[More water vapor over the Arctic has strong implications for Greenland. Anybody want to bet how much rain Swiss Camp gets in the next decade?]

I believe you have identified why the Northwest Passage or any other North Polar route for shipping will not become a preferred route for cargo ships, super tankers and anything that floats other than Coast Guard cutters and ice breakers.

Think of the massive surface area of a container ship drifting through fog banks and low hanging clouds for days on end while the surface moisture freezes and eventually accumulates so much weight the ship capsizes. Another tipping point; ice on board.

I do not tease. I took the NSIC ice data, dropped it into Excel, and calculated “anomalies” in terms of standard deviations on a month by month basis. (There are some differences between the area data and the extent data, so I added them together to smooth the curve.) Then, I plotted it. Because SD relates the size of the anomaly to the monthly average value and historical variance of the ice areas, such a plot provides a better feeling for whether the process is in control than say, CT’s “Tale of the Tape.” Since 2004, it looks like a process that is out of control and moving toward a new equilibrium. If you normalize the data, the effect is even more pronounced. (I did not.) I would say that we passed a tipping point for Arctic sea ice sometime before 2000.

Thus, we cannot assume the Arctic ice area or extent will tend back toward the 1979-2000 mean.

So, I lay a plastic ruler across the last couple years of SD data, and it looks like 2008 will be 6 SD under the 1979-2000 mean which would correspond to an ice area of ~2 MKM^2.

In absolute terms, another year of melt like last year would put us in that range. So, if we are not assuming a trend back to the mean, it is possible. Given changes in North Atlantic inflows, Arctic air temperatures, water vapor sources, storm mixing from the 2007 low ice season,. . . . it might even be likely. It may not be the most likely range, but it is a range that should be seriously considered in public policy planning.

Aaron, it is tempting to do as you have suggested and come up with some scary numbers, though my suspicion is that this is the ‘number cruncher’s’ approach (which I, too, indulge in). This is not to say that the current anomaly is not a concern, nor that next year’s won’t also be, but what you are doing is extrapolating from one year’s data, beyond the previous trend. This is one reason why I suspect William is not overly worried yet about losing his bet next year.

The NSIDC ‘report card’ summarises very effectively the processes which contributed to this year’s exceptional melt season. Beyond the numbers, we have to ask what the probability is of such conditions being replicated next year. Whilst it looks very likely that the Spring melt season will see less ice than normal (plausibly, less than ever recorded), and the existing ice will be thin and will vanish more easily, we can’t anticipate the unusual high pressure area, or the long-lasting temperature anomalies in excess of 15C over such large areas. In fact, if such an event were to recur, then we might have to look at a great deal of what has until recently been understood about the Polar climate drivers and consider the possibility that climate change has begun to alter the weather patterns in ways beyond any model’s expectations.

This is why, though I expect the anomalies to persist over Spring and next Summer to some degree, I am not confident about forecasting these levels until at least the Winter data has been collated and the seasonal anomaly calculated. I will agree with you that it does look like we’ve left one ballpark and entered a new one, but I don’t currently expect the anomaly to stay as low as it is now (-2.5Mkm2) throughout the coming months.

I do not particularly look to the funny atmospheric conditions of last summer. I look to indications that warm, salty water from the North Atlantic has been flowing into the Arctic Ocean mid and near surface water for the last several years. (My tipping point.) And, I think the GCM miss key salinity structures in the Arctic Ocean Surface Waters. Such structures are nonexistent in SH waters. I think the models miss some small-scale atmospheric structures that are unimportant to a GACM, but which are important to ice models.

If an implementation of a GCM says that an event can occur over the course of a decade, 40 years in the future, and similar activities begin to occur only couple years later, and continue for the next couple of years, then we have to consider that possible the time scale on the GCM was off by an order of magnitude. We have to ask ourselves, “Was there some important feedback that was not included?”

When a model fails to predict an event, which is 5.4 standard deviations off of the climate baseline, it says to me that the model is not adequately describing the process. Or, we could say, “The process is out of control!”

Neither the historical, statistical model of the Arctic climate, nor the GCM have described the recent Arctic climate. I think the Arctic Sea Ice is moving rapidly toward a new equilibrium point. The small recovery of 2006 does not give me any confidence that Arctic Sea Ice will stabilize or trend back toward the 1979-2000 baseline. (Which seems to be warmer than the conditions described in the journals of some early explorers and fur traders.)

Moreover, we are talking about global warming, which is about heat. Ice area is useful for calculating albedo, but when we talk about sea ice, we are talking volume. Today, Arctic ice is thinner, so that it takes less heat to melt a Km^2 of ice. (It does not “vanish”, it absorbs heat and melts. The heat stays in the system.) There is less fresh water floating on the surface of the Arctic Ocean and insulating the ice from the warmer near surface waters. Saltier surface water means different convection patterns. I stand by my estimate of 2 MKm^2 as the minima for next summer’s ice.

So what do you tell an engineer that must build structures that are legally required to survive a two hundred year return event storm?